1. Field of the Invention
The invention relates to acoustic surface wave convolvers and more particularly to apparatus therein for eliminating in the interaction region the distortive effects of dispersion and reflection.
2. Description of the Prior Art
Acoustic surface wave convolvers in the prior art, particularly the convolver disclosed in Applicant's U.S. Pat. No. 3,833,867 entitled "Acoustic Surface Wave Convolver with Bidirectional Amplification," and assigned to the assignee of the present invention, typically include an interaction region comprised of a semiconductor film overlay on a piezoelectric body. Moreover, the semiconductor film has traditionally been deposited on the body such that the plane defined thereby is coincidental with the plane defined by the parts traversed by the propagating acoustic surface waves. Since the interaction region represents a change in the medium through which the acoustic surface waves must traverse, and because the film-on-substrate geometry is dispersive, the respective velocities and wave vectors associated with each of the propagating acoustic surface waves must therefore change thereby inducing phase delays in the interaction region.
The dispersion of the surface waves can be expressed in terms of the parameter which is the fractional change in phase delay per unit change in frequency, i.e.,: EQU .gamma. = (1/.tau.) (.delta..tau./.delta.f). 1.
Over a bandwidth .DELTA.f, the deviation in time delay is .DELTA..tau., therefore, equation (1) may be rewritten as, EQU .gamma. = (1/.tau.) (.DELTA..tau./.DELTA.f). 2.
The time bandwidth, TB, of the device, which is a measure of the product of the transversal time of the signal through the interaction region and the bandwidth of the launching devices may be represented by: EQU TB = .tau..DELTA.f = .DELTA..tau./.gamma.. 3.
In order to maintain phase coherence across the output structure or interaction region, it is desirable to maintain the deviation in delay time .DELTA..tau. below 1 RF period, thus: EQU .DELTA..tau..ltoreq.1/f, and 4. EQU TB = .ltoreq. 1/.gamma.f 5.
Accordingly, it can be appreciated that the dispersion experienced in the interaction region seriously limits the usefulness of the device since the time bandwidth product TB is inversely proportional thereto.
The prior art acoustic surface wave convolvers with interdigital center structures have also been traditionally limited to operation in the non-degenerate mode, i.e., where the frequencies of the two input signals are unequal. Operation in this mode requires precise spacing of the electrode fingers such that: ##EQU1## where k.sub.3 is the wave number (or wave vector) of the sum frequency signal .omega..sub.3 developed in the convolver, k.sub.1 and k.sub.2 are the wave numbers (or wave vectors) of the two input signals at .omega..sub.1 and .omega..sub.2, respectively, and d is the center-to-center spacing between successive electrode fingers of the interdigital center structure. Moreover, each electrode finger must be disposed such that it lies normal to the direction of the wave vector k.sub.3 of the product or convolution signal. Since the electrode fingers of the interdigital structure are normal to the direction of travel of the propagating acoustic surface waves, the reflections therefrom will be directed along the same paths traversed by the propagating acoustic surface waves. The effect is to introduce a number of stop bands in the frequency response and thus significantly reduce the effective bandwidth of the convolver. These reflections are not coherent but nonetheless contribute to the spurious signal level in the convolution signal and they have proven to be very difficult to eliminate in the nondegenerate mode of operation.